1. Field of the Invention
This invention relates, in general, to photocopying apparatus and, more specifically, to prevention of toner image disruption when using a vacuum conveyor system for transporting sheets in copiers and printers.
2. Description of the Prior Art
Image producing apparatus, such as electrostatographic copiers and printers, often have a number of different process stations through which receivers, such as copy sheets, pass for processing. In order to move the sheets between the stations, various types of transport or conveyor systems are used according to conventional practice. One type widely used in the industry employs a vacuum transport system comprised of perforated conveyor belts disposed over openings in a vacuum plenum. There is a differential pressure exerted between the top surface of the sheet, which is exposed to the ambient, and the bottom surface of the sheet adjacent to the perforations of the belts and exposed to the vacuum of the vacuum plenum. This pressure differential causes the sheet to be attracted to and retained by the vacuum through the perforations of the belts, for vacuum belt transport, of the sheet, from one station to another station within the apparatus.
One frequent location for such a vacuum transport system is between the transfer station, where toned images are transferred to the o sheet, and the fixing or fusing station, where the toner, of the toned images, is melted and fused to the sheet to form a permanent image on said sheet. Depending on the application, the conveyor system between these stations either transports the sheet above or below the transport system, but with the image side of the sheet being opposite the belts as the toned sheets are transported to the fusing station for fusing.
While vacuum transport systems are excellent transport systems for conveying sheets from one station to the next, when used to convey toned sheets from a highly charged station such as the transfer station, to an uncharged station, such as the fusing station, disruption of the toned images on said sheets may occur as the sheets leave the transport system. The disruption is caused by an electrical field created by the charge or potential differential between the toner on the transported sheet and the induced charge on the conductive plenum of the vacuum transport system. This differential results from the toner having a charge as it leaves the transfer station, which charge induces an opposite charge on the conductive plenum as the sheet, containing the toned image, moves with the vacuum belts across the conductive plenum. This induced or image charge on the vacuum plenum has a tendency to attract or hinder the toner from moving with its carrier, the transported sheet, as the transported sheet moves toward the fusing station and away from the induced charge. This interaction between the charge of the toner and the charge of the plenum results in a shifting or disruption of the toner particles on the sheet.
One method customarily used to prevent the above attraction or shifting of the toner or toned image caused by such electrical field, is a corona discharge element located adjacent to where the sheet leaves the transport system. By subjecting the charge remaining on the toner to the corona discharge device, the charge on the toner is substantially eliminated. No electrical field, therefore, can be set up between the toner and the plenum that would cause toner disruption. A corona discharge device, however, takes up space in the apparatus, has objectionable ozone discharge, is costly and uses apparatus power.
Another problem with the above transport system is triboelectric charging of the vacuum plenum caused by the transported sheet and vacuum belts rubbing against the plenum, which if not dissipated, causes the surface of the plenum to attract dust and floating toner particles. The dust or toner particles then rub off onto the copy sheet.
In addition to the image disruption problem and the triboelectric problem of vacuum transport systems, such systems also have a tendency to cause wrinkling or creasing of the sheet being transported. The wrinkling is caused by the sheet being drawn into the gaps between the individual vacuum belts of the transport system by vacuum from vacuum ports located in said gaps, which are needed to initially attract and direct the leading edge of the sheet toward the transport system. If this wrinkling or creasing is left uncorrected, the creasing or wrinkling becomes permanent as the sheets pass into and through the fusing station.
Therefore, it is desirable, and an object of this invention, to provide a transport system which can smoothly and predictably control the movement of the sheets, while also controlling the electrical fields established between the sheets and the transport system to avoid any image disruption or wrinkling of the sheets as said sheets are transported between two process stations in a copier, printer, or like apparatus.